The IL-2/IL-2 receptor (IL-2R) has been studied intensively because of its potential function in the development and regulation of the immune system. The IL-2R alpha chain has been shown to be expressed on CD4-CD8- thymocytes and activated T and B cells. In this report, we show that IL-2R alpha is also expressed on precursor B cells in the bone marrow. Its expression is initiated by functional rearrangement and expression of Ig mu heavy chain gene and is down-regulated when immature B cells mature and express IgD. Its potential function in early B cell differentiation is discussed in comparison with its role in thymocyte differentiation.